301. CID-Mediated In Vivo Selection: Heterogeneity among Large Animal Models

Abstract

Animal models are indispensable tools for probing physiological processes, pathological perturbations, and for developing new therapies. Ultimately, animal models must provide useful guideposts for developing gene therapy approaches that are eventually proven effective in humans. Significant differences in results using various gene therapy animal models have been described. However, little or no information exists regarding differences that exist among the available large animal models. Here we evaluated, in the hemopoietic cells of baboons, a system for selection that has previously demonstrated activity in mice, dogs and humans. This system employs a derivative of the thrombopoietin receptor (F36Vmpl), that is conditionally activated in the presence of a small molecule drug called a chemical inducer of dimerization (CID). Whereas cultured mouse, dog and human hemopoietic cells all respond to the mpl signal, we observed only a minor and variable response to the mpl signal in the cultured cells of baboons. Similarly, we have noted significant CID-dependent rises in the frequency of transduced hemopoietic cells in mice and in dogs, however here we show that responses to CID administration in three baboons were modest and variable. A number of factors may account for the disparity between baboons and other species. First, we have noted that GFP expression is attenuated in the red cells of baboons transplanted with the F36VmplGFP vector compared to red cells derived from mice, dogs and humans transduced with the same vector. Since red cells expressing F36Vmpl respond especially well to CID administration in the mouse and dog, an inadequate level of F36Vmpl expression may account for the absence of a red cell response in the baboons. Of note, placing F36Vmpl directly downstream of the LTR (rather than downstream of the IRES) failed to generate more prominent hematological responses to CID administration. A second issue pertains to the susceptibility of the baboon cells to proliferate in response to the murine mpl signal. As noted above, significant CID dependent growth occurs in transduced hemopoietic cells from mice, dogs and humans in response to the murine mpl signal. Whether baboon cells would have responded better to signals emanating from baboon mpl is unknown, however it is noteworthy that mouse and human mpl are indistinguishable with respect to their ability to induce growth of transduced human cord blood cells. A third set of considerations pertains to the optimal concentration of CID necessary to trigger cell growth. Head-to-head comparisons of transduced dog, baboon and human CD34+ cells point to differences in the relative thresholds for triggering mpl-induced growth, and differences between the species with respect to the concentrations at which excessive amounts of CID inhibit cell growth due to excessive occupancy of the drug binding domains. Finally, we have noted major differences in the pharmacokinetic profiles of AP20187 in baboons compared to dogs. These findings have general implications for the evaluation and development of new strategies for gene therapy.